Wireless communications can generally be made more reliable by increasing the power level of the transmitter or by decreasing the transmission data rate to a more robust data rate. The transmit power levels, however, are typically limited by regulations and design constraints of the wireless devices. For example, most countries or regions have regulations that specify particular power level limits for each frequency band. In addition, design constraints generally limit the cost, size and power consumption of wireless devices.
A number of standards have been implemented or proposed that describe a set of minimum requirements that a wireless device must support in order to be compliant with the standard. The standards typically define, for example, signal constellation and frame formats. The IEEE 802.11 standard, for example, and the various extensions to the 802.11 standard, such as 802.11a, b and g, are standards for wireless LAN systems that operate in various frequency bands and provide for various data rates. For a detailed description of the IEEE 802.11 standard, see, for example, IEEE, “Supplement to Standard for Telecommunications and Information Exchange Between Systems—LAN/MAN Specific Requirements—Part 11: Wireless MAC and PHY Specifications: High Speed Physical Layer in the 5 GHz Band,” IEEE 802.11a-1999 (September, 1999).
The High Performance Radio Local Area Networks (HIPERLAN) Type 2 standard (HIPERLAN/2) is a standard proposed by the European Telecommunications Standards Institute (ETSI) for wireless LAN systems that operate in the 5 GHz band. The HIPERLAN/2 standard specifies a different set of data rates than the IEEE 802.11 standard. For a detailed description of the HIPERLAN/2 standard, see, for example, ETSI, “Broadband Radio Access Networks (BRAN): 5 GHz High Performance Radio Local Area Networks (HIPERLAN) Type 2, Harmonized EN Covering Essential Requirements of Article 3.2 of the R&TTE Directive,” ETSI EN 301 893, v1.2.2, (June, 2003).
In order to meet a given standard, a particular wireless device must support, among other requirements, the set of mandatory data rates. The selection of a particular available data rate by a given wireless device, however, is outside the scope of the standards. In general, there is an inverse relationship between the selection of a transmit power level and a corresponding transmission data rate. In addition, for a number of modulation schemes, higher data rates also require greater linearity in the power amplifier. Thus, to increase the transmit data rate, for example, there generally must be a corresponding decrease in the transmit power level. Likewise, to increase the transmit power level, there generally must be a corresponding decrease in the transmit data rate.
A number of regulatory bodies, including the Federal Communications Commission (FCC) in the United States, European Conference of Postal and Telecommunications Administrations (CEPT) in Europe and Ministry of Posts and Telecommunications (MPT) in Japan, have defined emission limits for various frequency bands. The emission limits typically differ from band to band and from region to region, and in some cases transmit power control is required. In addition, directional antenna gain must be considered to stay within the specified emission limits. As data rates increase, there is a more severe requirement on the tolerable transmit signal error and require more power stage linearity and back off in power level. While the wireless LAN standards, such as IEEE 802.11 and HIPERLAN/2, specify minimum and maximum transmit power levels and a maximum error in the transmitted signal constellation and receiver sensitivity level (which are both data rate dependent), the adaptation of the transmit power level or improving the error in the transmitted signal constellation or the receiver sensitivity is outside the scope of the standards.
A need therefore exists for a method and apparatus for automatic transmit power variation in wireless communication systems, such as wireless LANs. A further need exists for improved techniques for enhanced transmit power variation that can control the transmit power level to meet emission limits and to maintain the transmit power level within amplifier performance limits.